Instagram and some refactoring

I'm using compass and blueprint because I wanted their pretty buttons, so that's what all the extra CSS is for.

Since the join models don't have any functionality themselves, I changed the associations to has_and_belongs_to_many just to simplify our models and the way we save them.

InstagramPost and Tweet now inherit from InterwebPost, but the attributes still have twitter in the name.

I added SessionsController which just logs the user in and out of Instagram.